The present invention relates to improvement in vapor pump flow rate controlling, particularly as applied in vapor recovery type liquid fuel dispensers.
Vapor pumps are known, and one of the most prevalent vapor pump designs is known as a positive displacement type. These may be used in vapor recovery fuel dispensers, which have gained popularity in recent years as the appreciation of the need to reduce environmental pollution has increased.
In particular, vapor recovery fuel dispensers, such as gasoline dispensers, are equipped with add-on components to supplement the main task of dispensing liquid fuel from a fuel reservoir to a receptacle, such as a motor vehicle tank. The add-on components include a vapor recovery line and a vapor pump, with a vapor recovery line open in the region of the liquid dispensing nozzle. The pumping of liquid into, for example, a motor vehicle gas tank, forces vapor out of the gas tank, and the operation of the vapor recovery pump retrieves those vapors and sends them back to the gasoline reservoir, where they can recondense, rather than escaping to the atmosphere to form pollution.
It is known that the rate of flow of the vapor in the vapor recovery portion of the system should be selected to avoid two undesired conditions. First, a vapor flow rate which is too low will not retrieve all of the vapor, thus permitting pollution to go on. A vapor recovery flow rate which is too high will pull in air, along with the vapor. The oxygen component of the air, if allowed to build to a relatively high level, can cause a dangerously explosive mixture to exist in the fuel reservoir. Accordingly, the vapor flow rate is of critical concern. Several prior endeavors have focused on calculating what the desired flow rate ought to be. For example, U.S. Pat. No. 5,040,577 to Pope, the disclosure of which is hereby incorporated by reference, describes a vapor recovery system in which the speed of the vapor recovery pump is set by a microprocessor so its volumetric flow rate matches the volumetric flow rate of the liquid dispenser. In one embodiment, the volumetric flow of the vapor recovery pump is modified so as to maintain an expected pressure at its input.
In application Ser. No. 824,702, filed Jan. 21, l992, now U.S. Pat. No. 5,156,199, there is disclosed a system for selecting a desired vapor flow rate which is modified from the liquid flow rate, to account for thermal expansion or contraction of the vapor caused by heat exchange with the liquid.
U.S. Pat. No. 5,038,838 to Bergamini et al. discloses a system in which the vapor pump is continuously controlled to draw in a volumetric quantity of a vapor/air mixture equal to the volumetric quantity of fuel delivered, plus a possible excess of air.
However, none of the aforementioned systems disclose or suggest how to monitor the actual flow rate through the vapor pump and compensate for any deviations from a desired flow rate.
Such deviations may be introduced by various aspects of the vapor recovery fuel dispenser system, and may vary from one installation to another. Variations in components such a hose length, the presence of liquid in the vapor line, or dirt/particle deposits on the inside of the vapor lines influence the inlet vacuum and/or discharge pressure. This influence increases or decreases the vacuum and/or discharge pressure, which in turn affects the amount of vapor flow through the vapor recovery system. Other components of the system which might influence the pressure differential across the pump and its resultant flow rate are the presence of hose breakaways, smaller size vapor return piping, and the like.
Accordingly, there remains a need in the art for a method and apparatus for monitoring the flow rate through the vapor pump and correcting for any discrepancies between a desired and ascertained flow rate.